The composition of myofibrillar protein isoforms is a major determinant of a muscle's functional properties. Different isoforms are characteristic of a particular muscle type and isoform switching can occur as a result of disuse, training, or disease. Such changes in protein composition are often accompanied by structural remodeling of the myofibrils. This study will use the lobster claw-closer muscles as a model to understand the processes involved in isoform switching and myofibrillar remodeling. During juvenile development, the muscle fibers of the lobster claw undergo a predictable transformation from slow to fast fibers and vice versa, in the same animal. Isoform-specific probes will be used with Northern blots and in situ hybridization to study the exact timing of isoform switching. Next, standard molecular and microscopic techniques will be used to study isoform gene expression, protein composition, and ultrastructural organization of single transforming muscle fibers. Finally, direct electrical stimulation will be used to experimentally manipulate these processes. The long term goal of this study is to understand how regulatory factors, such as myogenin and MyoD, control the processes involved in muscular plasticity.